add #[allow(rustdoc::broken_intra_doc_links)] to subxt-codegen

[StackOverflowExcept1on]

We are developing gear protocol and have decided to enable flag that will convert all warnings to errors when the link in a document comment does not exist.

I think it makes sense to add this attribute in order not to get warnings when building documentation.
The cargo doc warning looks like this and it comes from the code generated by subxt-codegen

warning: unresolved link to `IdentityInfo`
    --> gsdk/src/metadata/generated.rs:2590:21
     |
2590 | /                     #[doc = "Provide a judgement for an account's identity."]
2591 | |                     #[doc = ""]
2592 | |                     #[doc = "The dispatch origin for this call must be _Signed_ and the sender must be the account"]
2593 | |                     #[doc = "of the registrar whose index is `reg_index`."]
...    |
2605 | |                     #[doc = "  - where `R` registrar-count (governance-bounded)."]
2606 | |                     #[doc = "  - where `X` additional-field-count (deposit-bounded and code-bounded)."]
     | |_______________________________________________________________________________________________________^
     |
     = note: the link appears in this line:
             
             - `identity`: The hash of the [`IdentityInfo`] for that the judgement is provided.
                                            ^^^^^^^^^^^^^^
     = note: no item named `IdentityInfo` in scope
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
     = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

[cla-bot-2021]

User @StackOverflowExcept1on, please sign the CLA here.

[jsdw]

[edit: whoops, commented rather than commited!]

[jsdw]

I'm ok with this being added to the codegen since it is likely to be a common case; I just added it to the "main" codegen path, too.